1. Field of the Invention
The present invention relates generally to a platen head assembly in an ink jet printer and, more specifically, to means for adjusting the relative position between a substrate, composed of a paper sheet, and the print head.
2. Description of the Related Art
Impact printers such as the daisy wheel, dot matrix or wire matrix have proven to be very reliable in the field of high speed printing devices. An example of an impact printer of the wire matrix type is disclosed in Bader, U.S. Pat. No. 4,088,215. In this type of printer, letters, numbers and symbols are formed from a series of dots produced by the impact of the ends of a plurality of wire elements or keys on a substrate. The wire elements forcefully strike the substrate, thus requiring a backstop for the moving element to position and support the substrate. Generally the backstop comprises a platen disposed within the printing system and so arranged to position the substrate between itself and the character keys.
A platen which is fixed relative to the printing unit is known as a rear referencing platen; a platen which is associated with a member which places the substrate surface that faces the printing unit at a fixed distance from the printing unit is known as a front referencing platen.
Ink jet printers wherein the ink within the print head is of the solid-ink or phase-change type, which may also be referred to as hot melt ink, are also presently in use. The phase change or hot melt ink of the type utilized in an ink jet is characteristically solid at room temperature. When heated, however, the ink changes to a liquid state and is propelled onto the substrate in a desired character pattern. The ink thereafter solidifies on the substrate in the prescribed pattern. Although, with ink jet printer systems, there is no physical contact between the print head and the substrate, rear referencing of the platen has hitherto been utilized.
Regardless of whether an impact printer or an ink jet printer is utilized, it is generally desirable to maintain a constant distance or print head gap between the substrate surface and the character keys or print head. Although the gap distance is important in impact printer systems, it is critical in ink jet printers, and especially when utilizing solid ink or phase change ink. For example, if the print head gap, generally defined as the distance between the ejection nozzle and the substrate, is initially set for a particular substrate thickness, a thinner substrate material would create a larger print head gap and a thicker substrate material would result in a smaller print head gap. A print head gap which is not properly adjusted can result in print characters which are virtually impossible to distinguish. For example, if the substrate is positioned too close to the ink jet head the liquid ink may solidify in illegible blotches due to its coming in contact with the substrate too soon. If the print head is too far away from the substrate the spray pattern will generally be too large and the characters formed will not legible.
In printer systems where the thickness of the substrate could vary, such as when a sheet of paper of one thickness is printed and thereafter it is necessary to print on an envelope of a different thickness, or in other situations, some mechanism for moving the platen to maintain the desired print head gap is usually provided.
It is therefore important, in those cases in which a plurality of different thicknesses of substrate may be printed upon, to provide a compensating device which acts to maintain the same relative distance between the print head and the surface of the substrate regardless of changes in the thickness of the substrate. Various such compensation devices are known in the prior art.
For example, Bader, U.S. Pat. No. 4,088,215 discussed above, discloses a mechanism for adjusting the print head to substrate distance by moving the platen relative to the print head. The Bader device operates by utilizing electronic circuitry including a Hall effect transducer, voltage comparators, and sensing means to measure the gap distance and mechanically adjust the position of the platen.
In Liles, U.S. Pat. No. 3,750,792, the print head is movable toward and away from the fixed platen, under the influence of a sensing finger, to maintain the desired print head - substrate distance.
One example of a front referencing platen associated with a dot matrix printer is disclosed in Manriquez, U.S. Pat. No. 4,227,819. The Manriquez patent discloses a free floating platen which is adjustable to accommodate variations in substrate thickness. To make adjustments, the platen itself is mechanically disconnected from the printing system and reoriented commensurate with the orientation of the lower surface of the substrate.
Another example of such a platen is disclosed in Kurihira, et al., U.S. Pat. No. 4,143,977. The Kurihira et al. patent discloses an adjustable platen comprising a plurality of separate platen units disposed longitudinally along the print line of the substrate. The adjustable units individually engage the substrate at different points to compensate for thickness variations across the length of the substrate. The individual adjustable platen units are connected to actuating arms which move the platen units under operator control to bias the substrate toward a guide plate through the utilization of leaf springs and tension springs.
Further examples of platens of this type are described in Kwan et al, U.S. Pat. No. 3,912,068; Polit, U.S. Pat. No. 4,620,807; Kurihira, et al., U.S. Pat. No. 4,184,780; and Lawter, et al., U.S. Pat. No. 4,422,782. These and the previously described devices for adjusting the platen position to maintain a constant gap distance each require complex mechanisms with multiple moving parts.
The front referencing arrangements described above all employ relatively complicated mechanisms for positioning the substrate and include some form of backing plate which supports the substrate directly in line with the print head. As a result, they add significantly to the manufacturing and maintenance costs of the printers in which they are installed.